maintainer-scripts: Fix jit documentation build with update_web_docs_git

scripts/update_web_docs_git -r 9.3.0 -d gcc-9.3.0
failed after the sourceware upgrade, there is no python-sphinx10 package and
python3-sphinx is new enough that the docs build succeeded.

2020-03-12  Jakub Jelinek  <jakub@redhat.com>

	* update_web_docs_git: Drop SPHINXBUILD=/usr/bin/sphinx-1.0-build.